G. Staikov

Underpotential alloy formation in the system Ag(hkl)/Cd2+

The underpotential deposition (UPD) of Cd on “real” and “quasi-ideal” silver single crystal surfaces of (111) and (100) crystallographic orientation as well as on polycrystalline silver substrate has been investigated in 0.5 M Na2SO4 supporting electrolyte solutions. At high underpotentials 100 < ΔE < 350 mV, the UPD is characterized by a quasi-reversible adsorption/desorption of Cd whereas at low underpotentials. ΔE < 50 mV, an increase of the anodic stripping charge with the polarization time is found due to the formation of an AgCd alloy at the substrate surface. The time dependence of this process can be described by a parabolic rate law, the rate constant of which is a function of ΔE and temperature T. Relatively low activation energies of about 70 kJmol−1 were determined from measurements at 293 ⩽ T ⩽ 338 K. The results are discussed in terms of a semi-infinite-linear diffusion model. The alloy formation process is assumed to be initiated by a place exchange between Ag substrate atoms and vacancy sites within a mobile Cd adsorbate layer thus forming the initial stage of a highly distorted AgCd alloy. The further growth will take place by the movement of Ag atoms through the vacancy-rich surface alloy and the simultaneous deposition of Cd at the interface AgCd/Cd2+.

Form and step distance of polygonized growth spirals

Abstract The forms and the step distances of polygomized spirals of growth arising from emergence points of screw dislocations are calculated taking into account that the propagation rate of steps depends on step length. A rather unexpected result is obtained showing that the ratio between the step distance d and the radius ρ c of the inscribed circle in the geometrical form of the nucleus is independent of the geometrical form (i.e. number of corners) of the spiral and equal to approximately 19, very near to the value of this ratio for a circular spiral calculated by Cabrera and Levine.

Underpotential—overpotential transition phenomena in metal deposition processes

The transition phenomena in metal deposition processes have been studied in the system Ag(hkl)/Pb2+ by potentiostatic pulse experiments from the underpotential to the overpotential range. The analysis of the current-time transients indicates a model of progressive nucleation and hemispherical diffusion to the growing 3-D crystallites. An important influence of the initial state in the underpotential range on the nucleation rate has been found. Furthermore, the nucleation rate strongly depends on the overvoltage and on the substrate nature. All experimental results are explained by a stepwise bulk phase formation process including rearrangement of adatoms in the 2-D adsorbed layer, formation of critical clusters and further rearrangement and growth forming epitaxially orientated 3-D crystallites. The nucleation process must be treated by an atomistic model due to the derived low number of atoms in the critical cluster.

STM studies of real and quasi-perfect silver single crystal surfaces used in electrochemical experiments

Abstract Real and quasi-perfect silver single crystal surfaces, which have been used intensively in different electrochemical investigations, are studied by STM to determine the surface profile on an atomic level. The measurements were carried out on chemically polished macroelectrodes and electrolytically grown microelectrodes in contact with air as well as under potentially controlled electrochemical conditions. The real macroelectrodes show a much higher surface corrugation than the quasi-perfect microelectrodes which have large atomically flat terraces separated by monoatomic steps. The possibility to observe the dynamics of monoatomic steps under anodic and cathodic polarization conditions on quasi-perfect single crystal surfaces is demonstrated. Nucleation and crystal growth processes are observed by means of in situ STM investigations of lead electrodeposition on Ag(111) macroelectrodes. The results demonstrate the possibility to study fundamentally the initial steps of electrocrystallization by in situ STM under electrochemical well-defined conditions.

Slow transformation phenomena of lead adsorbates on electrolytically grown Ag(111) and Ag(100) electrode surfaces

Abstract Previous study of underpotential deposits of Pb and Tl on chemically polished Ag(111) substrates has shown that under certain conditions slow structural transformations in the submonolayer coverage take place. Voltammetric measurements taken by us on electrolytically grown Ag(111) and Ag(100) electrodes reveal new features of the Pb-adsorbates. The rate of structural transformations on Ag(111) increases with the step density. This effect is not observed on Ag(100) but kinetics limitations attend the formation of the adsorbate. An attempt is made to explain these phenomena.

Metallionenadsorption und kathodische metallabscheidung—I. Thallium- und bleiabscheidung auf (111) und (100) silbereinkristallflächen

The morphology and kinetics of the cathodic thallium and lead deposition on Ag (111)- and Ag (100) single crystal faces was studied by potentiostatic pulse experiments. The formed crystallites are oriented with their close packed planes (0001) and (111) parallel to the substrat surfaces. There exists a strong correlation between the close packed crystallographic directions of the substrates [110] and those of the crystallites [1120] and [110] respectively. The experimental current—time-transients are discussed on the basis of various nucleation-growth-models. From the morphological results and from the dependencies of the current—time-transients on the substrate orientation as well as on the inital state at different underpotentials it can be concluded that the nucleation process is connected with reorganization phenomena of the metal ion adsorbate.

STM studies in underpotential—overpotential metal deposition

Abstract The deposition of lead on “real” silver single crystal faces in the so-called underpotential ( upd ) and overpotential ( opd ) ranges was studied by in situ STM with atomic resolution. The results are compared with previous electrochemical, morphological, and in situ EXAFS and GIXS investigations. The STM images on bare substrates show regular hexagonal and quadratic lattices for the (111) and (100) planes, respectively, without any reconstructions. At high coverages in the upd range, well-ordered lead adlayers are observed on both substrate orientations. The lead monolayer on Ag(111) was found to be incommensurate, close packed, and rotated by about 4–5° with respect to the substrate. This agrees with previous GIXS measurements and indicates internal compressive strain within the adlayer. In contrast, lead on Ag(100) forms a superlattice structure which has a much lower adatom density than a hexagonal close-packed layer. The symmetry and interatomic distances are close to a c (2 × 2)Pb overlayer. In the opd range, 3-D lead crystallites are formed. On both substrate orientations, the contact plane of the lead crystallites is the close-packed (111) plane. At low overpotentials, the lead crystallites on Ag(111) grow epitaxially, rotated by about 5° with respect to the substrate. On Ag(100), two different orientations of lead crystallites in parallel to both [110] crystallographic directions of the substrate are observed. The nucleation rate of lead depends not only on the overpotential, but also on the initial polarization state in the upd range using an upd  opd transient technique. Nucleation and growth may proceed either inside the imperfect and strained upd -adlayer or on top of it according to the Stranski-Krastanov mechanism.

Metal ion adsorption and electrocrystallization

Deposition of lead onto silver single crystal electrodes from perchlorate solutions has been studied in the undervoltage and low overvoltage regions employing twin-electrode thin layer and potential pulse techniques. Pseudo-Nernstian θ isotherms exhibiting continuous coverage steps were found. 2D nucleation was not identified to be a kinetically relevant process. The rate of 3D nucleation decreases with increasing θ, thus indicating major structural dissimilarities between the 2D layer and the 3D nuclei.

Substrate-induced strain of UPD monolayers and two-dimensional—three-dimensional transition in metal electrodeposition

Abstract Various reasons for the appearance of substrate-induced strain in underpotential deposition (UPD) monolayers on strongly attractive substrates are discussed. An expression for the dependence of monolayer compression on underpotential is obtained considering the two-dimensional film as isotropic and elastic. A theoretical model is proposed for the nucleation of the three-dimensional metal bulk phase taking into account the compressive strain of the UPD monolayer. It is shown that previous results from electrochemical measurements for lead deposition on Ag(111) as well as recent experimental data obtained by in-situ X-ray scattering and scanning tunnelling microscopy are in good agreement with the theoretical considerations.

Characterization of the surface structure of silver single crystal electrodes by ex situ and in situ STM

Abstract The surface characteristics of “real” and “quasi-perfect” silver single-crystal electrodes were studied by ex situ and in situ STM under atmospheric conditions and in electrolyte solutions, respectively. In both cases, atomically resolved STM images were obtained showing different surface structures. The atomic structures observed situ STM studies correspond exactly to the surface structures of Ag(100) and Ag(111) single-crystal faces. The ex situ STM studies revealed different structures of the topmost monoatomic layer, which can be attributed to a surface modification by oxygen from the ambient atmosphere. Spectroscopic data for the tunneling current as well as Auger and XPS measurements are in agreement with this assumption. It is found that the surface characteristics of silver single-crystal electrodes depend strongly on the preparation procedure and on the time of exposure in air.